Drilling apparatus mounted on wheels, the "carrying" or "vehicle" part of which is made up of a front chassis with two wheels and a rear chassis with two other wheels, the front chassis and the rear chassis being joined together by an intermediate articulation about an approximately vertical axis is already known. This articulation allows the apparatus to be steered, particularly when turning, the angle formed between the front and rear parts determining the radius of the turn.
In present-day embodiments of this kind, the front chassis is usually situated wholly in front of the axis of articulation to the rear chassis; this front chassis carries one or two drilling arms. The rear chassis is situated wholly behind the intermediate axis of articulation and, for its part, supports all of the a mechanical, electrical, hydraulic and other equipment of the drilling apparatus, in particular: oil reservoir, fuel tank, combustion engine, electric motors, water pump, compressor, electric cabinets, electric cable winder, etc. Depending on the embodiment, the driving and work station of the apparatus, where the operator is, may be carried by the front chassis or by the rear chassis. Examples of such drilling apparatus mounted on wheels, with an articulated chassis, are given in documents EP-A-0 203 907 and GB-A-2 230 807.
The current design of this apparatus has drawbacks, particularly when the apparatus is turning, the front chassis and the rear chassis then forming an angle which is permitted by the intermediate articulation. Because of its mass and its position, all the equipment carried by the rear chassis actually tends in such a situation to unbalance the apparatus, in particular reducing its lateral stability. To avoid destability of the apparatus, the angle of turning must remain relatively restricted. What is more, the current design entails passing numerous hydraulic lines through the intermediate articulation.
There is also known, from document U.S. Pat. No. 3,823,902, a drilling apparatus mounted on wheels, with an articulated chassis, in which the rear chassis is extended over the front chassis, by a part of the platform kind which supports the drilling arm or arms, and the driving and work station, produced in the form of a pivoting cab. In this embodiment, the front chassis carries only the two front wheels of the apparatus, and is thus restricted to a steering assembly. All the equipment, including the drilling arm or arms are, in this instance, carried by the rear chassis alone. This is therefore apparatus in which the distribution of equipment and of mass between the front and rear chassis is very unbalanced.
The configuration of such apparatus means that when this apparatus is turning, its front support points (points of contact of the front wheels with the ground) are shifted toward the inside of the turn, without the center of gravity of the apparatus shifting significantly. This leads to a risk of the apparatus toppling over toward the outside of the turn, and the instability of the apparatus is, in this instance, aggravated by the fact that its particular structure means that its center of gravity is higher up because the drilling arm or arms and the cab have to be high enough above the front wheels that they can pivot.
Thus, the apparatus according to document U.S. Pat. No. 3,823,902 also has a great deal of lateral instability when turning. What is more, the fact that the drilling arm or arms of this apparatus are carried by the rear chassis makes the apparatus very bulky when turning, because the arm or arms do not follow the direction taken by the front chassis, this forcing an additional movement of retracting the arm or arms.